Electrostatic application of powder material to solid dosage forms

ABSTRACT

In a method of electrostatically applying a powder material to a solid dosage form, charged powder material is applied to a photoconductive drum ( 3 ), is transferred to an intermediate belt ( 4 ) and then to a solid dosage form ( 5 ). The belt ( 4 ) makes contact with the solid dosage form ( 5 ).

The present invention relates to a method and apparatus for theelectrostatic application of powder material onto the surfaces of soliddosage forms, and more particularly, but not exclusively, pharmaceuticalsolid dosage forms.

A “solid dosage form” can be formed from any solid material that can beapportioned into individual units; it may be, but is not necessarily, anoral dosage form. Examples of pharmaceutical solid dosage forms includepharmaceutical tablets, pharmaceutical pessaries, pharmaceutical bougiesand pharmaceutical suppositories. The term “pharmaceutical tablet”should be interpreted as covering all pharmaceutical products which areto be taken orally, including pressed tablets, pellets, capsules andspherules. Examples of non-pharmaceutical solid dosage forms includeitems of confectionery and washing detergent tablets.

The electrostatic application of powder material to solid dosage formsis known. In the known techniques, the powder is generally applieddirectly onto the solid dosage forms, either by sprayingelectrostatically charged powder material onto the solid dosage forms,or by holding the powder material at a potential difference to the soliddosage forms sufficient to cause the powder material to be attracted tothe solid dosage forms. For example, WO92/14451 describes a process inwhich the cores of pharmaceutical tablets are conveyed on an earthedconveyor belt and electrostatically charged powder material is sprayedonto the tablet cores to form a powder coating on the exposed surface ofthe tablet cores. WO96/35516 describes a process in which the cores ofpharmaceutical tablets are held substantially isolated from theirsurroundings adjacent to a source of powder at a potential difference tothe tablet cores sufficient to cause the exposed surface of the tabletcores to become coated with the powder.

The present invention provides a method of electrostatically applying apowder material to a solid dosage form, the method comprising the stepsof electrostatically applying a powder material to a first intermediatemeans, and transferring the powder material that has been applied to thefirst intermediate means from the first intermediate means to the soliddosage form.

Applying the powder material to a first intermediate means before it isapplied to the solid dosage form has certain advantages. It becomespossible to provide an arrangement in which the location of thedeposition of the powder material can be closely controlled and, forexample, enables powder material to be deposited on a solid dosage formin a precise pattern. It may also facilitate the deposition of powdermaterial on a three dimensional surface.

Any suitable method may be used to apply the powder materialelectrostatically to the first intermediate means. For example, thefirst intermediate means may be earthed and the powder material held ata potential sufficient to cause the powder material to adhere to thefirst intermediate means.

In a preferred embodiment of the invention the powder material isapplied to the first intermediate means by applying an electrostaticcharge to the first intermediate means, and holding the powder materialat a potential sufficiently different from the potential of the firstintermediate means to cause the powder material to adhere to the firstintermediate means.

A first especially advantageous feature of a preferred embodiment of theinvention is that the electrostatic charge may be applied to the firstintermediate means in a pattern, making it possible to apply powdermaterial onto a solid dosage form in the form of a pattern. Any desiredpattern may be produced simply by applying a suitable electrostaticcharge pattern to the first intermediate means. Thus, it is, forexample, possible to print onto a solid dosage form the name or thedosage of the solid dosage form, or to apply to the solid dosage form alogo or some other design. By using different coloured powder materials,it is also possible to produce a pattern but at the same time have anuninterrupted coating on the solid dosage form. For example, differentcoloured powder materials could be used to produce a solid dosage formhaving a striped coating over all of the surface of a region of thesolid dosage form or over the whole of the solid dosage form.

Where a coating is applied to parts only of a region being coated, thecoating is referred to herein as discontinuous, even though in the caseof, for example, joined up writing each part of the coating may becontinuous with the other parts.

The electrostatic charge does not have to be applied to the firstintermediate means in a pattern. It may be applied to the firstintermediate means over the whole of a surface portion thereof.Accordingly, a conventional unpatterned and uninterrupted coating may beformed, if desired. Such a coating is referred to herein as a continuouscoating but it will be understood that it may or may not, for example,cover all of a surface of a solid dosage form.

In the case where an electrostatic charge is applied to the firstintermediate means, that means may be any means which is capable ofmaintaining an electrostatic charge on its surface. For example, thefirst intermediate means may be in the form of a drum or a belt and maycomprise a photo-conductive semi-conductor at its surface. Aphoto-conductive semi-conductor is a material which conducts electricityon exposure to light, but behaves as an insulator in the absence oflight. An electrostatic charge pattern may be applied to such a firstintermediate means by electrostatically charging the semi-conductor inthe dark, and then projecting an image onto the semi-conductor. Theelectrostatic charge will be dissipated in the illuminated areas, butwill be retained in the unilluminated areas. Thus, an electrostaticcharge pattern in the shape of the image will be formed on thesemi-conductor. Such first intermediate means are used in conventionalphotocopiers as photo-conductive drums or belts. For example, aphotoconductive drum used in the present invention may be a conductivedrum coated with selenium, selenium/arsenic or selenium/tellurium, or aconductive drum coated with a thin layer of photoconductive pigment in abinder resin, and a charge transport layer coated over thephotoconductive pigment layer. A photoconductive belt used for theinvention may be a flexible conductive substrate coated withphotogenerator layer comprising a photoconductive pigment in a binderpolymer overcoated with a charge transport layer.

The powder material should possess a defined electrostatic charge whichis either (a) of the same sign of charge as the residual charged areapattern on the photoconductive drum or belt after light exposure, or (b)of opposite sign of charge to the residual charged pattern on thephotoconductive drum or belt after light exposure. In the case (a) thepowder will be developed onto the areas of the photoconductive drum, orbelt, which have been discharged, i.e. the light illuminated areas, andwill be repelled by the areas of the photoconductive drum, or belt,which remain charged. Conversely in case (b) the powder will bedeveloped onto the areas of the photoconductive drum, or belt, whichremain charged, and will not be developed onto areas of thephotoconductive drum, or belt, which have been discharged, i.e. thelight illuminated areas. The powder material may have a permanent ortemporary net charge. Any suitable method may be used to charge thepowder material. Advantageously, the electrostatic charge on the powdermaterial is imparted by a triboelectric charging process (as is commonin conventional photocopying) or by corona charging.

Any suitable method may be used to apply the charged powder onto thefirst intermediate means. Methods have already been developed in thefields of electrophotography and electrography and examples of suitablemethods are described, for example, in Electrophotography andDevelopment Physics, Revised Second Edition, by L. B. Schein, publishedby Laplacian Press, Morgan Hill Calif.

A second especially advantageous feature of a preferred embodiment ofthe invention is that there is contact between the first intermediatemeans and the solid dosage form during transfer of the powder materialfrom the intermediate to the solid dosage form. Contact between thefirst intermediate means and the solid dosage form increases theaccuracy and speed and completeness with which the powder can betransferred to the dosage form. That may be advantageous irrespective ofthe method used to apply the powder material electrostatically to thefirst intermediate means. However, it is particularly advantageous wherethe powder material is applied in the form of a pattern.

The solid dosage form will, in general, be a three-dimensional object.For example, a conventionally-shaped pharmaceutical tablet comprises anupper domed surface and a lower domed surface, the two domed surfacesbeing joined together by an edge surface. In the known techniques wherepowder material is applied directly onto the solid dosage form, it isdifficult to obtain uniform application of powder material, especiallyto the edges of the solid dosage form.

Accordingly, preferably, the first intermediate means conforms partiallyor completely to the shape of the solid dosage form on transfer of thepowder material to the solid dosage form. In the case where the soliddosage form is a pressed tablet of domed shape the first intermediatemeans may conform only to the shape of the domed part of the tablet ormay also contact the cylindrical side wall of the tablet.

If the first intermediate means is able to conform to the shape of thesolid dosage form, it becomes possible to transfer powder material withgreater uniformity to the edges of the solid dosage form. That may beadvantageous irrespective of the method used to apply the powdermaterial to the first intermediate means. Where the powder material hasbeen applied to the first intermediate means in a discontinuous mannerto form a pattern, it also becomes possible to reduce or even eliminatedistortion of the pattern on transfer of the powder to the edges of thesolid dosage form.

Any suitable method may be used to transfer powder material from thefirst intermediate means to the solid dosage form. The powder materialthat adheres to the first intermediate means may be transferred from thefirst intermediate means to the solid dosage form, at least partly, byelectrostatic means. For example, the solid dosage form may be held at apotential sufficient to overcome the attractive forces of the powdermaterial to the first intermediate means, and to cause the powdermaterial to adhere to the solid dosage form instead. Alternatively, orin addition, the powder material that adheres to the first intermediatemeans may be transferred from the first intermediate means to the soliddosage form at least partly by heating the powder material during thetransfer, and/or at least partly by means of pressurised contact betweenthe first intermediate means and the solid dosage form.

If there is only one intermediate means, it must be possible to applypowder material electrostatically to that intermediate means, and tosubstantially transfer the powder material from that intermediate meansto the solid dosage form. However, the properties required for theelectrostatic application are not always compatible with the propertiesrequired for the transfer to the solid dosage form, particularly if thefirst intermediate means also has to be especially flexible.

Accordingly, a third especially advantageous feature of a preferredembodiment of the invention is that the powder material that has beenapplied to the first intermediate means is transferred from the firstintermediate means to the solid dosage form via a second intermediatemeans. The first intermediate means then requires only those propertieswhich are necessary for electrostatic application of the powder materialto the first intermediate means, and the second intermediate meansrequires only those properties which are necessary to enable powdermaterial to be transferred from the first intermediate means to thesecond intermediate means and from the second intermediate means to thesolid dosage form.

Advantageously, there is contact between the first intermediate meansand the second intermediate means on transfer of the powder materialfrom the first intermediate means to the second intermediate means.Advantageously, there is contact between the second intermediate meansand the solid dosage form on transfer of the powder material from thesecond intermediate means to the solid dosage form. More advantageously,the second intermediate means conforms partially or completely to theshape of the solid dosage form on transfer of the powder material to thesolid dosage form. The second intermediate means may be in the form of adrum or a belt and may comprise an elastomeric material, for example asilicone rubber, that may be sufficiently soft to deform as required.Elastomeric materials used for the construction of the secondintermediate means are, for example, rubber materials of defineddurometer hardness. Durometer hardness can be described by the Shore Ahardness scale. Materials particularly suitable would be, for example,silicone rubber with durometer hardness in the range 10A to 90A on theShore A scale.

Electrostatic forces may also cause or contribute to the transfer of thepowder material from the first intermediate means to the secondintermediate means and/or from the second intermediate means to thesolid dosage form.

Preferably, the method further comprises the step of treating the powdermaterial to fix it on the solid dosage form. Where the powder materialhas been applied in a continuous manner, the treatment may result in theformation of a continuous coating on the solid dosage form.

The treatment of the powder material to secure it to the solid dosageform preferably involves a heating step, preferably using convection,but other forms of heating such as infra red radiation or conduction orinduction may be used. The powder material should be heated to atemperature above its softening point, and then allowed to cool to atemperature below its glass transition temperature (Tg). Where thepowder material has been applied in a discontinuous manner, it may bedesirable to ensure that too much heat is not applied as the powdermaterial may spread once it has fused, and that may result in distortionor even loss of the pattern. It is also important to control the amountof heat applied to avoid degradation of the powder material and/or thesolid dosage form. The amount of heat required may be reduced byapplying pressure to the powder material during the transfer step.Alternatively, the powder material may include a polymer which is curedduring the treatment, for example, by irradiation with energy in thegamma, ultra violet or radio frequency bands.

The powder material may be treated to fix it on the solid dosage form asit is being transferred to the solid dosage form. For example, wherethere is contact between the solid dosage form and the firstintermediate means or the second intermediate means on transfer of thepowder material to the solid dosage form, fusing may be achieved byusing the first intermediate means or the second intermediate means toapply heat with or without pressure to the solid dosage form.Alternatively, the treatment may be carried out after the powdermaterial has been transferred to the solid dosage form.

The method may comprise the step of applying powder material to a firstsurface of the solid dosage form, and the subsequent step of applyingpowder material to a second surface of the solid dosage form. Such astep will usually be necessary if the whole surface of the dosage formis to be coated.

Preferably, the method is carried out as a continuous process.

The method of the present invention is not restricted to the use of anyparticular type of powder material. The powder materials described inPCT/GB96/01101 are examples of suitable powder materials.

The powder material may include an active material, for example abiologically active material, that is, a material which increases ordecreases the rate of a process in a biological environment. Thebiologically active material may be one which is physiologically active.

Conventionally, where an active material is to be administered in soliddosage form, the active material is mixed with a large volume ofnon-active “filler” material in order to produce a dosage form ofmanageable size. It has been found, however, that it is difficult tocontrol accurately the amount of active material contained in eachdosage form, leading to poor dose uniformity. That is especially thecase where the required amount of active material in each dosage form isvery low.

By electrostatically applying active material to a dosage form, it hasbeen found to be possible to apply accurately, and reproducibly, verysmall amounts of active material to the dosage form, leading to improveddose reproducibility.

The powder material comprising active material may be applied to a soliddosage form containing the same or a different active material, or maybe applied to a solid dosage form containing no active material.

The present invention further provides an apparatus forelectrostatically applying a powder material to a solid dosage form, theapparatus comprising means for applying a powder material to a firstintermediate means, and means for transferring the powder material thathas been applied to the first intermediate means from the firstintermediate means to the solid dosage form, or a means for transferringthe powder material that has been applied to the first intermediatemeans from the first intermediate means to a second intermediate meansand means for transferring the powder material subsequently from thesecond intermediate means to the solid dosage form.

The apparatus of the invention may be in a form suitable for carryingout the method of the invention in any of the forms described above.

By way of example, methods of electrostatically applying powder materialonto the surface of a pharmaceutical solid dosage form will now bedescribed with reference to the accompanying drawings in which

FIG. 1 shows schematically a first form of apparatus according to theinvention;

FIG. 2 is a diagrammatic view of the form of apparatus shown in FIG. 1and illustrating further features of the apparatus;

FIG. 2a is a view to a larger scale showing a detail of the apparatus ofFIG. 2; and

FIG. 3 is a diagrammatic view similar to FIG. 2 but showing a modifiedform of the apparatus of FIG. 2.

The apparatus shown schematically in FIG. 1 is for printing powdermaterial onto a single surface of a pharmaceutical pressed tablet. Theapparatus comprises a reservoir 1 for charged powder material.Downstream of the reservoir 1 is a rotatable developer roller 2 fortransferring charged powder material from the reservoir 1 to a firstintermediate means comprising a rotatable imaging drum 3 to which anelectrostatic charge pattern of opposite charge to the charge of thepowder material has been applied. The imaging drum 3 is aselenium-coated drum similar to those used in conventional photocopiers.Downstream of the imaging drum 3 is a second intermediate meanscomprising a rotatable intermediate belt 4 for transferring the powdermaterial that adheres to the imaging drum 3 to a pharmaceutical tablet 5carried on a conveyor belt 6. The intermediate belt 4 is able to conformto the cylindrical shape of the imaging drum 3 and also to the domedshape of the pharmaceutical tablet 5.

In use, an electrostatic charge is applied to the powder material as itleaves the reservoir 1. The developer roller 2 rotates, and as itrotates a layer of charged powder material is applied to its outersurface from the reservoir 1. An electrostatic charge of opposite chargeto the charge on the powder material is applied to the imaging drum 3 byelectrostatically charging the drum 3 in the dark. An image is thenprojected onto the drum 3 and the electrostatic charge dissipates in theilluminated areas, but is retained in the non-illuminated areas. Becausethe powder material is to be transferred from the imaging drum 3 to thepharmaceutical tablet 5 via the intermediate belt 4, the latentelectrostatic image should be a true image of the desired final pattern.The rotating developer roller 2 applies the charged powder material tothe imaging drum 3, which also rotates. The charged powder materialadheres to those parts of the imaging drum 3 to which an electrostaticcharge pattern has been applied, and retained. The intermediate belt 4rotates, and as it rotates it conforms to the shape of the imaging drum3 and the pharmaceutical tablet 5. In order to conform to the shape ofthe tablet 5, the belt 4 has to be able to curve about two orthogonalhorizontal axes. Powder material on the imaging drum 3 is transferred tothe intermediate belt 4 and then to the pharmaceutical tablet 5.Transfer from the imaging drum 3 to the intermediate belt 4 and from theintermediate belt 4 to the pharmaceutical tablet 5 can be promoted byapplying suitable electrical potentials to the belt 4 and thepharmaceutical tablet 5 at least in the region of each transfer. Thepowder material that has been applied to the pharmaceutical tablet 5will be in the pattern corresponding to the non-illuminated pattern onthe imaging drum 3 The conveyor belt 6 then carries the pharmaceuticaltablet 5 to a fusing station (not shown) where the powder material thathas been applied to the tablet 5 is fused and becomes fixed on thetablet 5.

While one particular embodiment of the invention has been described withreference to the drawings, it will be understood that many modificationsmay be made to the arrangement. For example, the imaging drum 3 may beheld at earth potential; in such a case the belt 4 and the roller 2 mayboth be held at, say, positive potentials. Also the intermediate belt 4may be in the form of a rotatable roller rather than a belt, and/or someother form of conveying means, apart from a conveyor belt, may beemployed.

As will be understood from the description above, it is also possiblefor the intermediate belt 4 to be omitted altogether, so that powderpasses from the imaging drum 3 directly to the tablets 5. Again, variousdifferent charging arrangements may be employed, including one in whichthe imaging drum 3 is held at earth potential and the development rollcharged to, say, a positive potential.

FIG. 2 illustrates some additional features of the same kind ofapparatus as that schematically illustrated in FIG. 1 and correspondingparts are referenced by the same reference numerals. Whereas in FIG. 1the tablets 5 are shown as being conveyed on a conveyor belt 6, in FIG.2 the tablets 5 are shown on the periphery of a tablet drum 16 which hasa plurality of tablet holders 17 around its periphery. The tablets 5 areretained on the tablet drum 16 by the application of reduced pressure tothe inner faces of the tablet, for example as described in WO96/35516,the description of which is incorporated herein by reference. As can beseen in FIG. 2a, the intermediate belt 4 is sufficiently flexible thatwhen brought into contact with the outer domed faces 5 a of the tablets5 it deforms, with the result that much of each domed face makes contactwith the belt 4.

FIG. 2 also shows the various stations around the imaging drum 3, bywhich powder is applied to the drum in a predetermined pattern. The drum3 is arranged to rotate anticlockwise as seen in FIG. 2 and the stationsthat the drum passes as it rotates anticlockwise as seen in FIG. 2 are acleaning station 18, a charging station 19, an exposing station 20 and adeveloping station 21. Each of the stations may be of a kind well knownper se in the field of electrophotography and their construction willnot be described in detail here.

In use, a region of the drum 3 that has just ceased contact with theintermediate belt 4 passes first to the cleaning station 18 where anypowder material still remaining on the drum is removed. That region ofthe drum 3 next passes to the charging station 19 where a uniformelectrostatic charge (opposite to the charge of the powder material tobe applied) is applied to the drum. Then, that region of the drum passesto the exposing station 20 where a pattern of light is projected ontothe drum, discharging the electrostatic charge from selected regions ofthe drum and leaving a pattern of charge on the drum, that chargepattern corresponding to the pattern in which the powder material is tobe applied to the tablets 5. Finally, at the developing station 21,charged powder material is applied, for example by the developer roller2, to the drum 3 and adheres to the portions of the drum on which theelectrostatic charge has been retained.

The patterned deposit of powder on the drum 3 travels round into contactwith the belt 4 and is transferred to the belt 4 with which it makesrolling contact. The pattern is then carried on the belt 4 (whichtravels in a clockwise direction as seen in FIG. 2) and into contactwith tablets 5 on the drum 16 as already described with reference toFIGS. 2 and 2a. After a tablet 5 has been carried into contact with thebelt 4 on the drum 16 it is carried away by the drum in a clockwisedirection as seen in FIG. 2 and may be carried past a fusing station 22,shown in dotted outline, where the powder material is fused and becomesfixed on the tablet 5.

If desired, the process described above with reference to FIGS. 2 and 2acan be repeated in order to coat the opposite domed faces of the tablets5.

FIG. 3 illustrates a modified form of the apparatus of FIG. 2 andcorresponding parts are referenced by the same reference numerals. Theapparatus of FIG. 3 performs the same functions as that of FIG. 2 butmost of the functions of the imaging drum 3 of FIG. 2 are performed bythe belt 4 of FIG. 2 and the imaging drum 3 is omitted. In the apparatusof FIG. 3, a cleaning station 18, a charging station 19 and a developingstation 21 are provided around the belt 4, but no exposing station ispresent. Thus the belt 4 is uniformly charged over its entire exposedface when it arrives at the developing station 21 and powder material istherefore deposited uniformly over the belt 4; the belt 4 rotatesclockwise and the tablet drum 16 rotates anticlockwise as seen in FIG. 3and powder material is transferred from the belt onto the domed faces ofthe tablets 5 as they come into contact with the belt 4. In this case,however, the powder coating is not patterned. As will be understood, thebelt 4 of FIG. 3 need not exhibit photo-conductive properties. Analternative arrangement, however, would be to employ a photo-conductivebelt as the belt 4 in FIG. 3 and furthermore to provide an exposingstation between the charging station 19 and the developing station 21;in that case a patterned layer of powder material could be applied tothe tablets 5.

If desired, the process described above with reference to FIG. 3 can berepeated in order to coat the opposite domed faces of the tablets 5.

What is claimed is:
 1. A method of electrostatically applying a powdermaterial to a solid dosage form, the method comprising the steps ofelectrostatically applying a powder material to a first intermediatemeans, and transferring the powder material that has been applied to thefirst intermediate means from the first intermediate means to the soliddosage form, wherein there is contact between the first intermediatemeans and the solid dosage form on transfer of the powder material tothe solid dosage form.
 2. A method according to claim 1, wherein thepowder material is applied to the first intermediate means by applyingan electrostatic charge to the first intermediate means, and applyingthe powder material at a potential sufficiently different from thepotential of the first intermediate means to cause the powder materialto adhere to the first intermediate means.
 3. A method according toclaim 2, wherein the electrostatic charge is applied to the firstintermediate means in a pattern.
 4. A method according to claim 2,wherein the first intermediate means comprises a photo-conductivesemi-conductor at its surface.
 5. A method according to claim 2, whereinthe powder material has an electrostatic charge opposite to theelectrostatic charge on the first intermediate means.
 6. A methodaccording to claim 5, wherein the electrostatic charge on the powdermaterial is applied by triboelectric charging or corona charging.
 7. Amethod according to claim 1, wherein the first intermediate meansconforms to the shape of the solid dosage form on transfer of the powdermaterial to the solid dosage form.
 8. A method according to claim 1,wherein the powder material that adheres to the first intermediate meansis transferred from the first intermediate means to the solid dosageform by means of the contact between the first intermediate means andthe solid dosage form.
 9. A method according to claim 1, wherein thepowder material that adheres to the first intermediate means istransferred from the first intermediate means to the solid dosage formby electrostatic means.
 10. A method of electrostatically applying apowder material to a solid dosage form, the method comprising the stepsof electrostatically applying a powder material to a first intermediatemeans, and transferring the powder material that has been applied to thefirst intermediate means from the first intermediate means to the soliddosage form, wherein the powder material that has been applied to thefirst intermediate means is transferred from the first intermediatemeans to the solid dosage form via a second intermediate means.
 11. Amethod according to claim 10, wherein there is contact between the firstintermediate means and the second intermediate means on transfer of thepowder material from the first intermediate means to the secondintermediate means.
 12. A method according to claim 10, wherein there iscontact between the second intermediate means and the solid dosage formon transfer of the powder material from the second intermediate means tothe solid dosage form.
 13. A method according to claim 12, wherein thesecond intermediate means conforms to the shape of the solid dosage formon transfer of the powder material to the solid dosage form.
 14. Amethod according to claim 1, wherein the method further comprises thestep of treating the powder material to fix it on the solid dosage formonce it has been transferred to the solid dosage form.
 15. A methodaccording to claim 1, wherein the method further comprises the step oftreating the powder material to fix it on the solid dosage form as it isbeing transferred to the solid dosage form.
 16. A method according toclaim 14, wherein the treatment of the powder material to secure it tothe solid dosage form results in the formation of a continuous coatingon the solid dosage form.
 17. A method according to claim 1, whichcomprises the step of applying powder material to a first surface of thesolid dosage form, and the subsequent step of applying powder materialto a second surface of the solid dosage form.
 18. A method according toclaim 1, wherein the method is carried out as a continuous process. 19.A method according to claim 1, wherein the powder material includes abiologically active material.
 20. A method according to claim 1, whereinthe solid dosage form is a pharmaceutical solid dosage form.
 21. Amethod according to claim 20, wherein the solid dosage form is for humanuse.
 22. A method according to claim 20, wherein the solid dosage formis a pharmaceutical tablet.
 23. An apparatus for electrostaticallyapplying a powder material to a solid dosage form, the apparatuscomprising means for applying a powder material to a first intermediatemeans, and means for transferring the powder material that has beenapplied to the first intermediate means from the first intermediatemeans to the solid dosage form, wherein the first intermediate means ispositioned such that there is contact between the first intermediatemeans and the solid dosage form on transfer of the powder material tothe solid dosage form.
 24. An apparatus according to claim 23, whereinthe first intermediate means comprises a photo-conductivesemi-conductor.
 25. An apparatus according to claim 23, wherein thefirst intermediate means is able to conform to the shape of the soliddosage form.
 26. An apparatus for electrostatically applying a powdermaterial to a solid dosage form, the apparatus comprising means forapplying a powder material to a first intermediate means, and means fortransferring the powder material that has been applied to the firstintermediate means from the first intermediate means to the solid dosageform, wherein the apparatus further comprises means for transferring thepowder material that has been applied to the first intermediate means toa second intermediate means, and means for transferring the powdermaterial that has been transferred to the second intermediate means tothe solid dosage form.
 27. An apparatus according to claim 26, whereinthe second intermediate means is positioned such that there is contactbetween the second intermediate means and the solid dosage form ontransfer of the powder material to the solid dosage form.
 28. Anapparatus according to claim 27, wherein the second intermediate meansis able to conform to the shape of the solid dosage form.
 29. Anapparatus according to claim 23, the apparatus further comprising meansfor treating the powder material to fix it on the solid dosage form onceit has been transferred to the solid dosage form.
 30. An apparatusaccording to claim 23, the apparatus further comprising means fortreating the powder material to fix it on the solid dosage form as it isbeing transferred to the solid dosage form.
 31. A coated substrateproduced by a method according to claim
 1. 32. An apparatus according toclaim 26, wherein the first intermediate means comprises aphoto-conductive semi-conductor.